Furthermore, we assume the quasi-static block-fading model is adopted for all channels involved in Fig. REF . Specifically, the baseband equivalent channels from the BS to the \(k\) th IRS, and from the \(k\) th IRS to the user are denoted by \({{\bf {u}}_k} = {\left[ {{u_{k,1}}, \cdots ,{u_{k,{M_k}}}} \right]^T}\in {\mathbb {C}^{{M_k} \times 1}}\) , and \({\bf {v}}_k^H = {\left[ {{v_{k,1}}, \cdots ,{v_{k,{M_k}}}} \right]^ * }\in {\mathbb {C}^{1 \times {M_k}}}\) , respectively, where \({u_{k,{m_k}}}\)  and \({v_{k,{m_k}}}\)  denote the channels from the BS and the user, respectively, to the \(m_k\) th element at the \(k\) th IRS. Let \({{\bf {\Phi }}_k} = \text{diag}\left( {{\varphi _{k,1}},{\varphi _{k,2}}, \cdots ,{\varphi _{k,{M_k}}}} \right) \in {\mathbb {C}^{{M_k} \times {M_k}}}\)  as the reflection coefficients matrix at the \(k\) th IRS, where \({{\varphi _{k,{m_k}}}}\)  denotes the reflection coefficient of the \({m_k}\) th element at the \(k\) th IRS. In this paper, we focus on the phase shift design by assuming the magnitude at each IRS element to be \(\left| {{\varphi _{k,{m_k}}}} \right| = 1\)  for the maximum reflection power collectionIn fact, the signal reflected by the IRS may experience a slight power loss for the IRS configuration. Besides, the magnitude of reflection coefficients may independently adjusted and/or coupled with the phase shift [1]}. The power allocation considering more practical IRS element model will be left for our future research topic.. In addition, we assume that the phase shift can be continuously adjusted in \(\left[ {0,2\pi } \right)\)  to gain essential insight. As a result, the effective channel from the BS to the user is modeled as the superposition of \(K\) -cluster reflection channels, i.e., \(h = \sum \limits _{k = 1}^K {{\bf {v}}_k^H{{\bf {\Phi }}_k}{{\bf {u}}_k}}\) .
